Lithography methods have been frequently used for the production of fine features in various kinds of electronic devices such as semiconductor devices and liquid crystal devices. At present, the photolithography method allows the formation of a fine resist pattern having a line width of about 90 nm. However, finer resist pattern formation will be required in future with miniaturization of device structures. Therefore, development of a photoresist composition that meets such needs as formation of a fine pattern has been an urgent task.
As the electronic device structure is miniaturized, further enhancement of resolution of projection optics has been also demanded. The resolution of projection optics is enhanced as the shorter exposure wavelength is employed, or as the numerical aperture of the projection optics is increased. Thus, the wavelength of light used in an exposure device has been lowered year by year, and with this lowering, numerical aperture of the projection optics has been increasing. Meanwhile, currently dominant exposure wavelengths involve 248 nm for KrF excimer laser, and further short wavelength of 193 nm for ArF excimer laser. When the exposure is carried out, focal depth is also important similarly to the resolution. The resolution R and the focal depth δ are represented by the following formulae, respectively.R=k1·λ/NA  (1)δ=±k2·nλ/(NA)2  (2)NA=n×sin θ  (3)
Wherein, λ represents an exposure wavelength; NA represents a numerical aperture of the projection optics; k1 and k2 represent a process coefficient; n represents a refractive index of a medium through which the exposure light passes; and θ represents an angle formed by the exposure light. When the medium through which the exposure light passes is air, n is 1. NA is theoretically up to less than 1, and is actually at most about 0.9 (θ=65°). From the above formula (1) and formula (2), it is proven that although the exposure wavelength λ may be shortened and the numerical aperture NA may be increased for enhancing the resolution R, decrease in the focal depth δ is accompanied thereby.
When the focal depth is excessively decreased, the area which can be focused on the substrate surface with respect to the imaging plane of the projection optics becomes narrow, whereby the area in which a favorable resist pattern can be provided also becomes narrow. As a result, production of electronic devices could be difficult. Thus, as a method to shorten the exposure wavelength, and to increase the focal depth, liquid immersion process was proposed (for example, see Patent Documents 1 and 2). This liquid immersion process not only enhances the resolution but increases the focal depth to about n (n: refractive index of a liquid being usually about 1.2 to 1.6) times by filling in between the bottom face of the projection optics and the substrate surface with a liquid such as water or an organic solvent to utilize the alteration of the wavelength of the exposure light in the liquid to 1/n as compared with that into the air.
Outline of one example of the exposure device for the liquid immersion process is explained with reference to FIG. 6. FIG. 6 shows a schematic construction of the exposure device for the liquid immersion process. Exposure device 20 for this liquid immersion process includes mask stage 22 for supporting mask 21, substrate stage 24 for supporting substrate 23, illumination optics 25 for illuminating the mask 21 with an exposure light, projection optics 26 for projection exposure of a pattern image of the mask 21 illuminated by the exposure light to substrate 23 supported by the substrate stage 24, and control apparatus 27 for controlling the entire operation of the exposure device 20.
The mask stage 22 executes positioning of the mask 21 supported on the mask stage 22 via mask stage driving apparatus 28 that is controlled by the control apparatus 27, while the substrate stage 24 executes positioning of the substrate 23 supported on the substrate stage 24 via substrate stage driving apparatus 29 controlled by the control apparatus 27. Although not shown in the figure, organic films such as a photoresist film and a protective film as in the present invention are mounted on the substrate 23.
With respect to the light source for exposure used in the illumination optics 25, for example, lines in ultraviolet region emitted from a mercury lamp (g-ray, h-ray, i-ray), KrF excimer laser beams (wavelength 248 nm), ArF excimer laser beams (wavelength 193 nm), F2 laser beams (wavelength 157 nm) and the like may be used. Of these, preferable light source provides ArF excimer laser beams. Reduced projection of the light passed through the mask 21 is conducted via the projection optics 26 to an exposure region on the substrate 23 at a predetermined projection magnification, whereby the exposure is performed.
In addition, the exposure device 20 for the liquid immersion process has liquid supply section 31 for supplying liquid immersion medium 30 on the substrate 23, and liquid recovery section 32 for recovery of the liquid immersion medium 30 on the substrate 23. This liquid immersion medium 30 is acceptable as long as it has a refractive index larger than the refractive index of air. The liquid supply section 31 and the liquid recovery section 32 are both controlled by the control apparatus 27, whereby the amount of liquid supply and the amount of liquid recovery on the substrate 23 per unit time are controlled.
In such exposure device 20 for the liquid immersion process, when, for example, pure water is used as the liquid immersion medium, the numerical aperture NA or the focal depth δ can be increased 1.44 times since pure water has a refractive index of 1.44 (in case of the light source providing ArF excimer laser (wavelength: 193 nm)). Therefore, when the liquid immersion process is adopted employing conventional projection optics, breakthrough of a barrier of a line width of 65 nm referred to as a limit in use of the ArF excimer laser is enabled without using F2 laser beams (wavelength: 157 nm), and it is said that microfabrication to the line width of 45 nm can be theoretically achieved.